send_result_set_metadata

Analysis
--------
Cursor inside trigger accessing NEW/OLD row leads server exit.

The reason for the bug was that implementation of function
create_tmp_table() was not considering Item::TRIGGER_FIELD_ITEM
as possible alternative for type of class being instantiated.
This was resulting in a mismatch between a number of columns
in result list and temp table definition. This mismatch leads
to the failure of assertion
DBUG_ASSERT(send_result_set_metadata.elements == item_list.elements)
in the method Materialized_cursor::send_result_set_metadata
in debug mode.

Fix:
---
Added code to consider Item::TRIGGER_FIELD_ITEM as valid
type while creating fields.